A field effect transistor has been widely used as a unit electronic device of a semiconductor memory integrated circuit, a high-frequency signal amplifier device, a liquid crystal drive device, and the like. The field effect transistor is an electronic device that is most widely put to practical use at present.
In recent years, development of displays has rapidly progressed, and a thin film transistor (TFT) has been widely used for a display (e.g., liquid crystal display (LCD), electroluminescence (EL) display, or field emission display (FED)) as a switching device for driving a display by applying a driving voltage to the display element.
A silicon semiconductor compound is most widely used as a material for the thin film transistor. A silicon single crystal is generally used for a high-frequency signal amplifier device, an integrated circuit device, and the like for which a high-speed operation is required, and amorphous silicon is generally used for a liquid crystal drive device and the like for which an increase in area is required.
However, since a high temperature (e.g., 800° C. or more) is required to crystallize a crystalline silicon thin film, it is difficult to form a crystalline silicon thin film on a glass substrate or an organic substrate. Therefore, a crystalline silicon thin film can only be formed on an expensive heat-resistant substrate (e.g., silicon wafer or quartz). Moreover, a large amount of energy and a large number of steps are required to produce a crystalline silicon thin film.
Since the device structure of a thin film transistor (TFT) that utilizes a crystalline silicon thin film is normally limited to a top-gate structure, it is difficult to reduce cost by reducing the number of masks, for example.
An amorphous silicon semiconductor (amorphous silicon) thin film can be formed at a relatively low temperature, but can achieve a low switching speed as compared with a crystalline silicon thin film. Therefore, when using an amorphous silicon semiconductor thin film for a switching device for driving a display, a high-speed animation may not be displayed.
A silicon-based semiconductor film is mainly used for a switching device that drives a display. This is because a silicon-based thin film exhibits excellent stability, excellent workability, a high switching speed, and the like. A silicon-based thin film is generally formed by chemical vapor deposition (CVD).
A thin film transistor (TFT) may have an inverted staggered structure in which a gate electrode, a gate insulating layer, a semiconductor layer (e.g., hydrogenated amorphous silicon (a-Si:H)), a source electrode, and a drain electrode are stacked on a substrate (e.g., glass substrate). In the field of large-area devices (e.g., image sensors), a thin film transistor (TFT) having an inverted staggered structure is used as a driver device of a flat panel display (e.g., active matrix liquid crystal display). When using amorphous silicon in such an application, an increase in operation speed has been desired along with enhancement in functionality.
In view of the above situation, an oxide semiconductor thin film that exhibits excellent stability as compared with a silicon-based semiconductor thin film has attracted attention.
However, it is difficult to industrially apply a transparent semiconductor thin film formed of a metal oxide (particularly a transparent semiconductor thin film formed by crystallizing zinc oxide at a high temperature) due to low field-effect mobility (hereinafter may be referred to simply as “mobility”) (about 1 cm2/·sec), a small on-off ratio, and occurrence of leakage current.
A crystalline oxide semiconductor that contains zinc oxide has been extensively studied, but has the following problems when film formation is conducted by sputtering which is normally conducted on an industrial scale.
For example, an oxide semiconductor film formed of a conductive transparent oxide that contains zinc oxide as the main component tends to produce a large number of carrier electrons due to oxygen defects. This makes it difficult to reduce conductivity. Moreover, an abnormal discharge occurs during film formation by sputtering, so that the film formation stability is impaired. This results in deterioration in uniformity and reproducibility of the resulting film.
Therefore, when using an oxide semiconductor film formed of a conductive transparent oxide that contains zinc oxide as the main component as an active layer (channel layer) of a thin film transistor (TFT), for example, a large amount of current flows between the source electrode and the drain electrode even if the gate voltage is not applied (i.e., a normally-off operation cannot be implemented). It is also difficult to increase the on-off ratio of the transistor.
Moreover, the resulting thin film transistor (TFT) may exhibit poor TFT characteristics (e.g., low mobility, small on-off ratio, a large amount of leakage current, unclear pinch-off, and tendency to be normally-on state), and may not be produced by wet etching due to poor chemical resistance. Therefore, the production process and the usage environment are limited.
An oxide semiconductor film formed of a conductive transparent oxide that contains zinc oxide as the main component must be formed under high pressure (i.e., the film-forming rate decreases) in order to improve the characteristics, and requires a high-temperature process (700° C. or more). This makes industrial production difficult. A thin film transistor (TFT) that utilizes an oxide semiconductor film formed of a conductive transparent oxide that contains zinc oxide as the main component exhibits poor TFT characteristics (e.g., mobility) when using a bottom-gate structure. It is necessary to employ a top-gate structure and increase the film thickness to 100 nm or more in order to improve the TFT characteristics.
In order to solve the above problems, use of an amorphous oxide semiconductor film that contains indium oxide, gallium oxide, and zinc oxide for a thin film transistor has been studied. Production of an amorphous oxide semiconductor film that contains indium oxide, gallium oxide, and zinc oxide by sputtering that ensures high mass productivity on an industrial scale has also been studied. However, gallium is a trace metal, and increases the raw material cost. Moreover, the characteristics (e.g., mobility and S value) of the transistor deteriorate when a large amount of gallium is added.
Patent Document 1 and Non-patent Document 1 disclose a thin film transistor that utilizes an amorphous oxide semiconductor film that contains indium oxide, tin oxide, and zinc oxide, but does not contain gallium. A sputtering target that is used for an optical information recording medium and contains tin oxide as the main component has been studied (Patent Document 2). However, practical applications of a sputtering target for forming an oxide semiconductor have not been specifically studied.
An effect of reducing the number of tin aggregates has been studied in order to suppress generation of nodules of an ITO target for forming a transparent conductive film (Patent Document 3). However, the most excellent target contains tin aggregates in an amount of about 2.6 per mm2, and an effect of further reducing the number of tin aggregates in an oxide semiconductor application has not been studied.